1. Field of the Invention
This invention relates to a magnesia clinker, particularly a sea water magnesia clinker, having excellent corrosion resistance, which is suitable as the starting material for a refractory for steel manufacturing furnace, particularly a refractory for converter such as magnesia carbon refractory.
2. Description of the Prior Art
Marked progresses have been noted in recent developments of steel manufacturing techniques, and corresponding developments have been desired also for refractory for steel manufacturing furnaces. Thus, with the progress in steel manufacturing techniques, the refractory to be used for steel manufacturing furnace is required to be resistant under harsh conditions. Accordingly, expensive materials such as electrofused magnesia, which has scarcely been used in the prior art as the starting material for refractory for steel manufacturing furnace, are coming to be employed. Particularly, in the magnesia carbon refractory to be used as the refractory for converter, electrofused magnesia is frequently employed, and therefore less expensive calcined magnesia as substitute therefor (particularly sea water magnesia clinker) has earnestly been sought after. Sea water as herein mentioned refers only to sea water in the oceans in general, and bittern and brine are excluded therefrom.
The magnesia clinker suitable as the starting material for magnesia carbon refractory has been said to be one having large crystal sizes of magnesia, a high bulk density and a high MgO purity, and among those satisfying those conditions is particularly desired a magnesia clinker having large crystal sizes. Accordingly, as the magnesia source for magnesia carbon refractory, there was an attempt to use a magnesia clinker having a mean crystal diameter of about 80 .mu.m alone or in combination with electrofused magnesia.
The term "clinker" as herein mentioned refers to a mass which is formed by melting the portion with lower melting points of the components generally by sintering and then solidifying the whole. In the prior art, there is well known the so called electrofused magnesia which is prepared by converting magnesia clinker, etc. into completely fused state by electrical heating, followed by solidification. However, as can be understood from the manufacturing step, electrofused magnesia is small in production amount and expensive. Accordingly, as a substitute material for electrofused magnesia, there had been a demand for magnesia clinker of large crystals and high bulk density. Whereas, as different from electrofused magnesia, the clinker prepared by calcination will inevitably contain minute amounts of impurities and pores and, due to the presence of such impurities and pores, there was involved the drawback that the crystal could hardly be grown.
Clinker is produced generally by molding magnesium hydroxide formed through the reaction between calcium hydroxide and sea water (or brine) as such or molding the powdery caustic magnesium obtained by calcining magnesium hydroxide or natural magnesite once at a temperature from 600.degree. to 1000.degree. C., followed by calcination at a high temperature of 1900.degree. C. or higher. The magnesia clinker obtained by calcination had crystal sizes of 20 to 40 .mu.m at the largest. Some magnesia clinker prepaed from the starting material of a magnesite containing large amounts of Fe.sub.2 O.sub.3 and SiO.sub.2 have crystal sizes of 60 .mu.m or larger. However, any of these magnesia clinkers had a purity less than 97%.
As the method for increasing the crystal size of MgO in magnesia clinker, there is the method in which purity of MgO is enhanced and the method in which ZrO.sub.2 is used as the additive, as disclosed in Japanese Laid-open Patent Publication No. 104054/1983. The method for enhancing MgO purity comprises hydrating magnesia calcined once at 800.degree. C. to 1400.degree. C., removing calcium oxide and calcining again the product, thereby increasing the purity of MgO to 99.5% or higher to give a magnesia clinker with high bulk density and large crystals. Even by this method, it is not possible to obtain a magnesia clinker having crystal sizes of 100 .mu.m or larger, and besides the production cost is increased to a great extent due to the complicated step of removing calcium oxide.
Concerning the method employing an additive, a magnesia clinker having a mean crystal size of 60 .mu.m or larger prepared by addition of ZrO.sub.2 is disclosed in the pending patent application, but it was difficult to obtain a magnesia clinker with a mean crystal size of 100 .mu.m or larger only by addition of ZrO.sub.2.